1. Field of the Invention
This invention relates to a power source circuit for a flash discharge lamp which is required for fixing toner on the electrostatic recording paper of a facsimile, printer, etc. by irradiation from the flash discharge lamp.
2. Description of the Prior Art
One prior art power source circuit for a discharge flash lamp comprises a booster-rectifier including a transformer and a full-wave rectifier, a thyristor connected to the booster-rectifier on the side of the power source, a discharge capacitor connected to the booster-rectifier on the side of a load, a detecting element for detecting a voltage across the discharge capacitor, a trigger circuit, and a control circuit for controlling the thyristor. The thyristor is phase controlled by the control circuit to cause the voltage of the discharge capacitor to have a predetermined value. This circuit is set forth, for example, in Japanese Utility Model Disclosure No. 89870/77 "Power Source Device for Flash Discharge Lamp".
FIG. 1 illustrates the above-mentioned conventional power source circuit. Reference numeral 1 indicates an AC power source; 50 designates a charging circuit including a resistor 8 for detecting a voltage across a discharge capacitor 6. The charging circuit 50 comprises a transformer 3, a rectifier 4, a charging resistor 5 and other resistors provided as required. Trigger circuit 51 includes a trigger switch 12, resistors 9 and 10, a capacitor 11 and a pulse transformer 13. Reference numeral 2 denotes a thyristor connected between the AC power source 1 and the charging circuit 50; 52 represents a thyristor control circuit, which includes a comparator 19 for receiving a signal from the above-mentioned resistor 8, a capacitor 21 for activating a unijunction transistor 23, a transformer 15, a resistor 16, a rectifier 17, a Zener diode 18 for voltage stabilization use, a pulse transformer 24 and resistors 20 and 22. Pulse signals a and b from the thyristor control circuit 52 serve as control signals for the thyristor 2.
In the above described power source circuit, a control circuit 53, for the capacitor 21 for activating the unijunction transistor 23, includes a switch 25 ganged with the trigger switch 12 and a transistor connected to the above-mentioned capacitor 21. A drive circuit 54 cooperates with and drives the control circuit 53.
The control circuit 53 and the drive circuit 54 are shown in detail in FIG. 2. In FIG. 2, reference numeral 25 indicates the switch ganged with the aforementioned trigger switch; 55 designates a commercially available monostable circuit; and 56 identifies a differentiation circuit. When the switch 25 is closed, a drive circuit for the transistor 26 is formed. The drive circuit comprises the differentiation circuit 56 and the monostable circuit 55, and it operates to generate a pulse having a pulse width which is dependent upon the time constants of a capacitor C.sub.4 and a resistor R.sub.6 at the output stage of the monostable circuit 55. During the pulse, the transistor 26 is held in its conductive state. Accordingly, if the collector of the transistor 26 is connected, via a terminal 53c, to the capacitor 21, no charges are stored therein during the duration of the pulse, so that the signal generation from the thyristor control circuit 102 is stopped, resulting in the thyristor 2 becoming nonconductive. Thus, occurrence of the glow discharge state can surely be prevented.
The drive circuit 54 has input terminals 54a and 54b, output terminals 54c and 54d and a Zener diode 27 for output voltage stabilization use and functions as a DC stabilizing power source and is connected to input terminals 53a and 53b of the capacitor control circuit 53.
In this conventional power source device, when the trigger switch 12 is closed to cause the flash discharge lamp 14 to radiate, the drive circuit 54, the capacitor control circuit 53 and the thyristor 52 operate to retain the thyristor 2 in its nonconductive state for a certain period of time, thereby insuring that glow discharge of the flash discharge lamp is prevented.
However, according to package tests of actual toner fixation in an electronic copying machine, it has been found that noise generation is very great in the above-mentioned conventional power source device. A close examination shows that when conducting the thyristor for controlling its phase, a large amount of noise is generated at an abrupt fall in the voltage waveform.